Computer Networks: Multiplexing

Prof. Taek M. Kwon

Department of Electrical Engineering, UMD

For EE1001

Outline

EE 4321 Multiplexing NFC

EE 4321: Computer Networks

• EE Technical Elective Course, 3 credits

• Network Lab (MWAH 60)

• Course Objective: to learn characteristics of network transmission media, protocol architectures (OSI, TCP/IP), data link protocols, routing algorithms, various LAN technologies, WAN technologies, and network programming.

Multiplexer (Mux)

y

x0

x1

x2

x3

S1 S0

S1 S0

y

0 0 x0 0 1 x1 1 0 x2 1 1 x3

Demultiplexer (Demux)

y

x0

x1

x2 x3

S1 S0

S1 S0

x3

x2

x1

x0

0 0 - - - y 0 1 - - y - 1 0 - y - - 1 1 y - - -

Mux/Demux

Multiplexing

Multiplexing Forms

Frequency Division Multiplexing (FDM) – A number of signals are carried simultaneously on the same medium by allocating to each signal a different frequency band.

Time Division Multiplexing (TDM) Synchronous TDM – Each signal source is

interleaved using a fixed time slot assigned. Statistical TDM -- Time slots are not preassigned.

Rather, data are transmitted using the available time slots.

Frequency Division Multiplexing

FDM System

Wavelength Division Multiplexing

Multiple beams of light at different frequency Carried by optical fiber A form of FDM Each color of light (wavelength) carries separate data

channel 1997 Bell Labs

100 beams Each at 10 Gbps Giving 1 terabit per second (Tbps)

Lab systems (Alcatel) 256 channels at 39.8 Gbps each 10.1 Tbps Over 100km

Frequency Hopping Spread Spectrum (FHSS)

signal is broadcast over seemingly random series of frequencies

receiver hops between frequencies in sync with transmitter

eavesdroppers hear unintelligible blips jamming on one frequency affects only a

few bits

Frequency Hopping Example

Code Division Multiple Access (CDMA)

a multiplexing technique used with spread spectrum

given a data signal rate D break each bit into k chips according to a

fixed chipping code specific to each user resulting new channel has chip data rate

kD chips per second can have multiple channels superimposed

CDMA Example

Synchronous Time Division Multiplexing (TDM)

Data rate of medium exceeds data rate of digital signal to be transmitted

Multiple digital signals interleaved in time May be at bit level of blocks Time slots preassigned to sources and

fixed Time slots allocated even if no data Time slots do not have to be evenly

distributed amongst sources

Time Division Multiplexing

TDM System

TDM Link Control No headers and trailers Data link control protocols not needed Flow control

Data rate of multiplexed line is fixed If one channel receiver can not receive data, the

others must carry on The corresponding source must be quenched This leaves empty slots

Error control Errors are detected and handled by individual channel

systems

Data Link Control on TDM

Digital Carrier Systems

Hierarchy of TDM USA/Canada/Japan use one system ITU-T use a similar (but different) system US system based on DS-1 format Multiplexes 24 channels (64Kbps/channel) Each frame has 8 bits per channel plus one

framing bit 193 bits per frame (24x8 +1 =193) Single voice channel =8KHz (Nyquist rate=2B,

4Kx2=8K Samples per sec, 8bit per sample)

Digital Carrier Systems (2) For voice each channel contains one word of

digitized data (PCM, 8000 samples per sec) Data rate 8000x193 = 1.544Mbps (193=24x8+1) Five out of six frames have 8 bit PCM samples Sixth frame is 7 bit PCM word plus signaling bit Signaling bits form stream for each channel containing control

and routing info

Same format for digital data 23 channels of data

• 7 bits per frame plus indicator bit for data or systems control

24th channel is sync

Digital Carriers (TDM Hierarchy)

Digital Signal Designation

Bandwidth/data rate

Channels (DS0s)

Carrier designation

DS0 64 kbps 1

DS1 1.544 Mbps 24 T1

DS1C 3.152 Mbps 48 T1c

DS2 6.312 Mbps 96 T2

DS3 44.736 Mbps 672 T3 = 7xT2

DS4 274.176 Mbps 4032 T4 = 6xT3

DS5 400.352 Mbps 5760 T5 = 60 xT2

SONET/SDH (1)

Synchronous Optical Network (SONET) An optical transmission interface originally

proposed by Bellcore and standardized by ANSI

Synchronous Digital Hierarchy (SDH) SONET compatible standard published by

ITU-T

SONET/SDH (2)

Signal Hierarchy Synchronous Transport Signal level 1 (STS-1)

or Optical Carrier level 1 (OC-1)=51.84Mbps, 51.88x106/65K=810 DS0 channels

STS-3/OC-3, 155.52Mbps=51.84x3Mbps STS-192/OC-192, 9953.28Mbps=

51.84x192Mbps STS-768/OC-768, 40Gbps = 51.84x768Mbps

= 622,080xDS0 ITU-T lowest rate is 155.52Mbps (STM-1)

Statistical TDM

In Synchronous TDM many slots are wasted

Statistical TDM allocates time slots dynamically based on demand

Multiplexer scans input lines and collects data until frame full

Data rate on line lower than aggregate rates of input lines

Synchronous Vs. Statistical TDM

A B D C A B D C A B D C Synchronous TDM

A B D A Statistical TDM

C

Statistical TDM Frame Formats

Performance

Output data rate is less than aggregate input rates (the average amount of input is less than the capacity of the multiplexed line).

May cause problems during peak periods Buffer inputs Keep buffer size to minimum to reduce delay

Buffer Size and

Delay

Cable Modem Outline Two channels from cable TV provider dedicated to data

transfer One in each direction Each channel is shared by a number of subscribers

Scheme needed to allocate capacity Statistical TDM

Cable Modem Operation Downstream

Cable scheduler delivers data in small packets If more than one subscriber active, each gets fraction of

downstream capacity • May get 500kbps to 20Mbps

Also used to grant (allocate) upstream time slots to subscribers

Upstream User requests timeslots on shared upstream channel. Dedicated

slots are allocated for this request. Headend scheduler sends back assignment of future time slots

to subscriber using downstream

Cable Modem Scheme

NFC Near-field communications, an extremely short-range wireless technology, will soon revolutionize payment and access systems.

NFC Applications

Using an NFC-enabled phone to exit a London train station

Using NFC to pay parking

NFC Applications

Using an NFC-enabled phone to ride a bus in China

Japan’s subway entry

NFC Applications

Tokyo, a candy store

Near/Far Field

Far field: the orthogonal electric and magnetic fields extend out from the antenna beyond several wavelengths. The field strength decreases by a factor of 1/d2,

Near field: the field is within one wavelength or less from the antenna. Magnetic field is more dominant. The signal strength decreases by 1/d6.

NFC Modes

NFC Applications

Mobile payment device: restaurants, parking lots, theaters, ports stadiums, buses, taxies, airlines,…

Electronic key: home access, secure buildings, car doors, computers,…

Pairing: peer-to-peer data exchange Reading of smart stickers or tags: provides

specifications, features, price, sales, web link, etc.

NFC Chips for Projects, AS3911